In the printing technology it is known to separate out sheets from a stack of sheets individually and to feed them to a printing machine. For this purpose a wide variety of configurations of sheet feeders are known which generally feed sheets of even thickness. One such configuration has a lifting apparatus having a substantially flat support for accommodating a stack of sheets and a feed device disposed thereover. By means of the lifting apparatus the sheets of the stack of sheets are lifted into the region of the feed device and then received and conveyed away by the latter.
In some applications, however, the sheets to be fed are of different thicknesses in their end regions lying laterally to the feed direction. These sheets can for example carry a substrate to be printed. It is also possible for the sheet to have a punched out region which is held in position e.g. by means of adhesive tape, the sheet then having a different thickness in the region of the adhesive tape in comparison to the rest of the sheet. If this type of sheet having different thicknesses in the end regions is to be fed, the conventional sheet feeders can not be used because the upper side of the stack of sheets would be inclined if it were to lie on a horizontal support. Therefore, in the past special sheet feeders were developed for such applications. For example, U.S. patent application Ser. No. 2006/0284366 A1 shows a curved sheet support which is capable of accommodating a stack of sheets comprising sheets of different thicknesses in their end regions such that the uppermost sheet lies flat. By means of the support's special pivoting technology it is possible for the thicker end region of the stack of sheets to be lifted more quickly than the thinner end region so that the respective uppermost substrate remains flat and horizontal.
U.S. Pat. No. 6,283,469 B1 shows a sheet feeder which has a fixed horizontal support component and a second support component, which is pivotable in relation to the first support component. The pivotable component can be pivoted below the plane of the horizontal component in order to accommodate the region of the sheets having a greater thickness. By pivoting the pivotable component in relation to the horizontal component it is possible during the sheet feeding process to lift the thicker region of the stack of sheets more quickly than the component which remains horizontal so as to provide a substantially flat and horizontal alignment of the respective uppermost sheet of the stack of sheets.